Implications of OMV including Debian versus Debian plus OMV

I came back here to have a look at OMV after quite some time and I am impressed by the development it has taken.

I am currently running a custom install of Debian Jessie with ZFS on Linux as my home server, but looking at easy configuration (web gui) and easy addition of features (plug-ins) I wonder whether OMV might be a better setup. ZFS is a requirement, by the way, but I am really only comfortable around Linux, so the likes of FreeNAS/NAS4Free/ZFSguru are not an alternative.

My somewhat general question is: How intrusive or how modest does OMV behave in relation to its Debian base? Will a custom Debian with OMV installed as a package on top of it work equally well as a default install from the OMV iso? Or will it turn that Debian install upside down like openATTIC does when you install it? Will I win or lose some features if I have my own partition setup, (sudo) users, ssh with byobu, etc. configured beforehand? Or can I have all the Debianness in an OMV install, like adding KVM and TV card drivers (no gui or media center needed).

Also, will plug-ins conflict with other parts of an extended Debian install? Where, for example, is the difference between setting up ZFS on Linux on standard Debian and OMV's ZFS plug-in? Will both reflect snapshots and other functions of ZFS inside the web gui?

These questions are difficult to answer from all the guides and material I've read about OMV, so I would be delighted to hear from the experienced.

OMV does take over configs of some services like ssh. It will never change partitions or even fstab. But if you don't mount a filesystem with OMV (other than zfs), you can't use it in the web interface.

If you are this experienced with Linux, you should like the combination once you figure out what OMV controls and what you can control. My best recommendation is try the things you want to try in a VM.